This invention relates to compositions that are suitable for use as adhesives in microelectronic devices or semiconductor packages.
Adhesive compositions, particularly conductive adhesives, are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. The more prominent uses are the bonding of integrated circuit chips to lead frames or other substrates, and the bonding of circuit packages or assemblies to printed wire boards.
The requirements for conductive adhesives in electronic packaging are that they have good mechanical strength, curing properties that do not affect the component or the carrier, and thixotropic properties compatible with existing application equipment currently used in the industry.
Another important aspect of an adhesive bonding or interconnection technology is the ability to rework the bond. For single chip packaging involving high volume commodity products, a failed chip can be discarded without significant loss. However, it becomes expensive to discard multi-chip packages with only one failed chip; consequently, the ability to rework the failed chip would be a manufacturing advantage. Today, one of the primary thrusts within the semiconductor industry is to develop adhesives that will meet all the requirements for adhesive strength and flexibility, but that will also be reworkable, that is, will be capable of being removed without destroying the substrate.
This invention is an adhesive composition for use in electronic devices that comprises one or more mono- or polyfunctional maleimide compounds, or one or more mono- or polyfunctional vinyl compounds other than maleimide compounds, or a combination of maleimide and vinyl compounds, a curing initiator, and optionally, one or more fillers. The composition can be designed to be reworkable.
In another embodiment, this invention is the cured adhesive that results from the just described curable adhesive composition.
In another embodiment, this invention is a microelectronic assembly comprising an electronic component bonded to a substrate with a cured adhesive composition prepared from a composition comprising one or more mono- or polyfunctional maleimide compounds, or one or more mono- or polyfunctional vinyl compounds, or a combination of maleimide and vinyl compounds, a curing initiator, and optionally one or more fillers.
The maleimide and vinyl compounds used in the adhesive compositions of this invention are curable compounds, meaning that they are capable of polymerization, with or without crosslinking. As used in this specification, to cure will mean to polymerize, with or without crosslinking. Cross-linking, as is understood in the art, is the attachment of two polymer chains by bridges of an element, a molecular group, or a compound, and in general will take place upon heating. As cross-linking density is increased, the properties of a material can be changed from thermoplastic to thermosetting.
It is possible to prepare polymers of a wide range of cross-link density by the judicious choice and amount of mono- or polyfunctional compounds. The greater proportion of polyfunctional compounds reacted, the greater the cross-link density. If thermoplastic properties are desired, the adhesive compositions can be prepared from mono-functional compounds to limit the cross-link density. A minor amount of poly-functional compounds can be added to provide some cross-linking and strength to the composition, provided the amount of poly-functional compounds is limited to an amount that does not diminish the desired thermoplastic properties. Within these parameters, the strength and elasticity of individual adhesives can be tailored to a particular end-use application.
In those cases where it is necessary to rework the assembly and thermoplastic materials are used, the electronic component can be pried off the substrate, and any residue adhesive can be heated until it softens and is easily removed.
The cross-link density can also be controlled to give a wide range of glass transition temperatures in the cured adhesive to withstand subsequent processing and operation temperatures.
In the inventive adhesive compositions, the maleimide compounds and the vinyl compounds may be used independently, or in combination. The maleimide or vinyl compounds, or both, will be present in the curable package adhesive compositions in an amount from 2 to 98 weight percent based on the organic components present (excluding any fillers).
The adhesive compositions will further comprise at least one free-radical initiator, which is defined to be a chemical species that decomposes to a molecular fragment having one or more unpaired electrons, highly reactive and usually short-lived, which is capable of initiating a chemical reaction by means of a chain mechanism. The free-radical initiator will be present in an amount of 0.1 to 10 percent, preferably 0.1 to 3.0 percent, by weight of the organic compounds (excluding any filler). The free radical curing mechanism gives a fast cure and provides the composition with a long shelf life before cure. Preferred free-radical initiators include peroxides, such as butyl peroctoates and dicumyl peroxide, and azo compounds, such as 2,2xe2x80x2-azobis(2-methyl-propanenitrile) and 2,2xe2x80x2-azobis(2-methyl-butanenitrile).
Alternatively, the adhesive compositions may contain a photoinitiator in lieu of the free-radical initiator, and the curing process may then be initiated by UV radiation. The photoinitiator will be present in an amount of 0.1 to 10 percent, preferably 1 to 5.0 percent, by weight of the organic compounds (excluding any filler). In some cases, both photoinitiation and thermal initiation may be desirable. For example, the curing process can be started by UV irradiation, and in a later processing step, curing can be completed by the application of heat to accomplish a free-radical cure.
In general, these compositions will cure within a temperature range of 80-200xc2x0 C., and curing will be effected within a length of time of less than 1 minute to 60 minutes. As will be understood, the time and temperature curing profile for each adhesive composition will vary, and different compositions can be designed to provide the curing profile that will be suited to the particular industrial manufacturing process.
Suitable conductive fillers for the adhesives are silver, copper, gold, palladium, platinum. In some circumstances, nonconductive fillers may be needed, for example to adjust rheology, such as, alumina, silica, and teflon.
As used throughout this specification, the notation C(O) refers to a carbonyl group.
The maleimide compounds suitable for use in the adhesive compositions of this invention have a structure represented by the formula: [Mxe2x80x94Xm]nxe2x80x94Q , or by the formula: [Mxe2x80x94Zm]nxe2x80x94K. For these specific formulae, when lower case xe2x80x9cnxe2x80x9d is the integer 1, the compound will be a mono-functional compound; and when lower case xe2x80x9cnxe2x80x9d is an integer 2 to 6, the compound will be a poly-functional compound.
[Mxe2x80x94Xm]nxe2x80x94Q, or by the formula: [Mxe2x80x94Zm]nxe2x80x94K.
Formula [Mxe2x80x94Xm]nxe2x80x94Q represents those compounds in which:
M is a maleimide moiety having the structure 
xe2x80x83in which R1 is H or C1 to C5 alkyl;
each X independently is an aromatic group selected from the aromatic groups having the structures (I) through (IV): 
Q is a linear or branched chain alkyl, alkyloxy, alkylene, alkyleneoxy, aryl, or aryloxy alkyl amine, alkyl sulfide, alkylene amine, alkylene sulfide, aryl sulfide species, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to X;
or Q is a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms; R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents; X is O, S, N, or P; and v is 0 to 50;
or Q is an ester having the structure: 
xe2x80x83in which R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents;
or Q is a siloxane having the structure:
xe2x80x94(CR12)exe2x80x94[SiR4xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94
xe2x80x83in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is H, an alkyl group having 1 to 5 carbon atoms or an aryl group, and e and g are independently 1 to 10 and f is 1 to 50; and
m is 0 or 1, and n is 1 to 6.
Preferred compositions are aliphatic bismaleimides in which the maleimide funtionality is linked to the backbone through urethane or urea linkages, such as in the following preferred compounds: 
Formula [Mxe2x80x94Zm]nxe2x80x94K represents those compounds in which
M is a maleimide moiety having the structure 
xe2x80x83in which R1 is H or C1 to C5 alkyl;
Z is a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to K;
or Z is a urethane having the structure: 
xe2x80x83in which each R2 independently is an alkyl, aryl, or arylalkyl group having 1 to 18 carbon atoms, and R3 is an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents, and v is 0 to 50;
or Z is a siloxane having the structure:
xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94
xe2x80x83in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is H, an alkyl group having 1 to 5 carbon atoms or an aryl group, and a and g are independently 1 to 10 and f is 1 to 50;
K is an aromatic group selected from the aromatic groups having the structures (VI) through (XIII) (although only one bond may be shown to represent connection to the aromatic group K, this will be deemed to represent any number of additional bonds as described and defined by n): 
xe2x80x83in which p is 1 to 100; 
xe2x80x83in which p is 1 to 100; 
in which R5, R6, and R7 are a linear or branched chain alkyl, alkyloxy, alkyl amine, alkyl sulfide, alkylene, alkyleneoxy, alkylene amine, alkylene sulfide, aryl, aryloxy, or aryl sulfide species, which may contain saturated or unsaturated cyclic or heterocyclic substituents pendant from the chain or as part of the backbone in the chain, and in which any heteroatom present may or may not be directly attached to the aromatic ring; or R5, R6, and R7 are a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CH3)gxe2x80x94in which the R1 substituent is H or an alkyl group having 1 to 5 carbon atoms and the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, and e is 1 to 10 and f is 1 to 50; 
xe2x80x83and m is 0 or 1, and n is 1 to 6.
Preferred maleimide compounds, particularly for reworkable compositions, are N-butylphenyl maleimide and N-ethylphenyl maleimide.
The vinyl compounds (other than the maleimides herein) suitable for use in the adhesive compositions of this invention will have the structure: 
For these specific structures, when lower case xe2x80x9cnxe2x80x9d is the integer 1, the compound will be a mono-functional compound; and when lower case xe2x80x9cnxe2x80x9d is an integer 2 to 6, the compound will be a poly-functional compound.
In these structures, R1 and R2 are H or an alkyl having 1 to 5 carbon atoms, or together form a 5 to 9 membered ring with the carbons forming the vinyl group; B is C, S, N, O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, C(O)NH or C(O)N(R8), in which R8 is C1 to C5 alkyl; m is 0 or 1; n is 1-6; and X, Q, Z, and K are as described above.
Preferably, B is O, C(O), C(O)xe2x80x94O, C(O)NH or C(O)N(R8); more preferably B is O, C(O), Oxe2x80x94C(O), C(O)xe2x80x94O, or C(O)N(R8).
The preferred vinyl compounds for use as adhesives are vinyl ethers or alkenyl sufides. Examples of suitable vinyl compounds are the following: 
Depending on the nature of the substrate to which the adhesive is to be bonded, the adhesive may also contain a coupling agent. A coupling agent as used herein is a chemical species containing a polymerizable functional group for reaction with the maleimide and other vinyl compound, and a functional group capable of condensing with metal hydroxides present on the surface of the substrate. Such coupling agents and the preferred amounts for use in compositions for particular substrates are known in the art. Suitable coupling agents are silanes, silicate esters, metal acrylates or methacrylates, titanates, and compounds containing a chelating ligand, such as phosphine, mercaptan, and acetoacetate. When present, coupling agents typically will be in amounts up to 10 percent by weight, and preferably in amounts of 0.1-3.0 percent by weight, of the maleimide and other monofunctional vinyl compound.
In addition, the adhesive compositions may contain compounds that lend additional flexibility and toughness to the resultant cured adhesive. Such compounds may be any thermoset or thermoplastic material having a Tg of 50xc2x0 C. or less, and typically will be a polymeric material characterized by free rotation about the chemical bonds, the presence of ether groups, and the absence of ring structures. Suitable such modifiers include polyacrylates, poly(butadiene), polyTHF (polymerized tetrahydrofuran), CTBN (carboxy-terminated butadiene-acrylonitrile) rubber, and polypropylene glycol. When present, toughening compounds may be in an amount up to about 15 percent by weight of the maleimide and other monofunctional vinyl compound.
If siloxane moieties are not part of the maleimide or vinyl compound structure, siloxanes can be added to the package formulations to impart elastomeric properties. Suitable siloxanes are the methacryloxypropyl-terminated polydimethyl siloxanes, and the aminopropyl-terminated polydimethylsiloxanes, available from United Chemical Technologies and others.
Other additives, such as adhesion promoters, in types and amounts known in the art, may also be added.
These compositions will perform within the commercially acceptable range for die attach adhesives. Commerically acceptable values for die shear for the adhesives on a 80xc3x9780 mil2 silicon die are in the range of greater than or equal to 1 kg at room temperature, and greater than or equal to 0.5 kg at 240xc2x0 C., and for warpage for a 500xc3x97500 mil2 die are in the range of less than or equal to 70 xcexcm at room temperature.
The coefficient of thermal expansion (CTE) is the change in dimension per unit change in temperature for a given material. Different materials will have different rates of expansion. If the CTE is very different for elements attached together, thermal cycling can cause the attached elements to bend, crack, or delaminate. In a typical semiconductor assembly, the CTE of the chip is in the range of 2 or 3 ppm/xc2x0 C.; for organic circuit board substrate, the CTE is greater than 30 ppm/xc2x0 C.; therefore, the CTE of the adhesive is best between that of the substrate and die.
When a polymer is subjected to the application of heat, it will move through a transition region between a hard, glassy state to a soft, rubbery state. This region is known as the glass transition region or Tg. If a graph of expansion of the polymer versus temperature is plotted, the glass transition region is the intersection between the lower temperature/glassy region coefficient of thermal expansion and the higher temperature/rubbery region coefficient of thermal expansion. Above this region, the rate of expansion increases significantly. Consequently, it is preferred that the glass transition of the polymer be higher than normal operating temperatures experienced during the application, and if reworkability is needed, that the glass transition be lower than any rework temperature.
Another embodiment of this invention includes the maleimides having the formulae [Mxe2x80x94Xm]nxe2x80x94Q and [Mxe2x80x94Zm]nxe2x80x94K as described herein in which Q and Z can be an ester having the structure 
or the structure 
in which p is 1 to 100,
each R3 can independently be an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents, or
a siloxane having the structure xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94CR12)gxe2x80x94in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms, the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, e and g are independently 1 to 10 and f is 1 to 50.
Another embodiment of this invention includes the vinyl compounds having the structures 
as described herein in which B is C, S, N, O, C(O), C(O)NH or C(O)N(R8), in which R8 is C1 to C5 alkyl.
Another embodiment of this invention includes the vinyl compounds having the structures 
as described herein in which Q and Z can be an ester having the structure 
or the structure 
in which p is 1 to 100,
each R3 can independently be an alkyl or alkyloxy chain having up to 100 atoms in the chain, which chain may contain aryl substituents,
or a siloxane having the structure
xe2x80x94(CR12)exe2x80x94[SiR42xe2x80x94O]fxe2x80x94SiR42xe2x80x94(CR12)gxe2x80x94
in which the R1 substituent independently for each position is H or an alkyl group having 1 to 5 carbon atoms, the R4 substituent independently for each position is an alkyl group having 1 to 5 carbon atoms or an aryl group, e and g are independently 1 to 10, and f is 1 to 50.
Another embodiment of this invention includes the curable adhesive composition as described herein containing an anionic or cationic curing initiator. The types and useful amounts of such initiators are well known in the art.